1. Field of the invention
The invention relates to controlling a power supply voltage provided to a microprocessor, and more particularly, to a circuit that automatically switches between supplying the microprocessor with a first voltage or a second voltage.
2. Description of the Related Art
Over the past decade, the performance and speed of microprocessors have improved dramatically. The number of transistors that can be fitted onto a microprocessor chip number is in the millions. Such densities are allowed primarily with the use of complementary metal oxide silicon (CMOS) technology, which allows for significantly lower power consumption over prior bipolar and NMOS technologies. However, even with the use of CMOS technology, power dissipation by today's high performance microprocessors is a significant problem. To reduce power consumption, microprocessors such as the 486SL and Pentium.RTM. CPUs from Intel Corporation include built-in power management functions. The energy efficient CPUs include a stop clock and auto halt mode. In stop clock mode, the clock speed of the microprocessor can be reduced or even stopped altogether, which reduces the power dissipation of the microprocessor when it is idle. The CPU enters into the auto halt mode after execution of a halt instruction. This allows the microprocessor to be placed into low power mode via software.
Neither of the above features, however, address how the power consumption of a microprocessor can be reduced while it is active. One popular method of reducing power consumption while the microprocessor is running is to reduce its power supply voltage from the industry standard 5 volts down to 3.3 volts. Lower voltages have also proved to be necessary as die sizes continue to shrink. The small dimensions that exist on a chip require a lower operating voltage to avoid problems associated with high current densities. With the trend towards 3.3 volt microprocessors chip manufacturers have had to provide both microprocessors that operate at 5 volts and 3.3 volts.
This presents the computer manufacturers with a problem, however, as the computer system must be capable of providing two different power supply voltages to the microprocessor. One solution is to include a 5-volt power supply voltage on all computer systems, adding a conventional regulator for providing a 3.3 volt power supply voltage in the 3.3-volt computer systems. However, this requires that the manufacturing process be different for 3.3-volt and 5-volt computer systems. Single-part solutions exist where the same part can be used with both 3.3-volt and 5-volt microprocessors. However, these parts include linear regulators, which are relatively expensive components. Thus the use of the separate, conventional linear regulator is undesirable, as it adds cost to the manufacture of the computer systems for several reasons.